Flaky test: AttachContainerTest.test_attach_no_stream

We've seen this test fail in moby CI (disabled it at some point https://github.com/moby/moby/pull/39848, but it looks to be still flaky after re-enabling it recently when we updated to the latest docker-py version https://github.com/moby/moby/pull/40467)

[2020-02-10T23:40:44.429Z] =================================== FAILURES ===================================
[2020-02-10T23:40:44.429Z] __________________ AttachContainerTest.test_attach_no_stream ___________________
[2020-02-10T23:40:44.429Z] tests/integration/api_container_test.py:1250: in test_attach_no_stream
[2020-02-10T23:40:44.429Z]     assert output == 'hello\n'.encode(encoding='ascii')
[2020-02-10T23:40:44.429Z] E   AssertionError: assert b'' == b'hello\n'
[2020-02-10T23:40:44.429Z] E     Right contains more items, first extra item: 104
[2020-02-10T23:40:44.429Z] E     Use -v to get the full diff
[2020-02-10T23:40:44.429Z] ------- generated xml file: /src/bundles/test-docker-py/junit-report.xml -------

Perhaps this needs a change similar to https://github.com/docker/docker-py/commit/ef043559c4bbd3d1fbc06277160c253fab6df879 (https://github.com/docker/docker-py/pull/2307)

Start with tests/integration/api_container_test.py at AttachContainerTest.test_attach_no_stream and reproduce the CI failure where output is empty instead of b'hello\n'. Review the referenced docker-py commit and pull request for a comparable change. Done means the test reliably receives the expected output without remaining flaky.